It is known that one of the most common problems found in rolling processes, in particular of billets and blooms, is connected to the need to keep the section area of the emerging product constant.
It is also known that the measurement is influenced by the possible presence of a drawing condition in the rolled product, between adjacent pairs of stands (hereafter referred to as inter-stand drawing).
It is also known that, in order to obtain a rolled product with substantially constant section sizes along its whole longitudinal extension it is necessary to keep the rolled product in an optimum condition of inter-stand drawing which, in some cases, is zero or almost zero.
In order to limit the variations in section size of the rolled product, a device is known, called loop former, which is disposed between two successive rolling stands and which allows to keep the inter-stand drawing at a value close to zero. In this way variations in the section area of the rolled product due to the inter-stand drawing are avoided, and also the accumulation of rolled product between the two rolling stands, which might entail a risk of jamming.
The loop former, however, does not allow to correct, by acting on the inter-stand drawing, the variations in section area of the rolled product that are generated by other causes.
The presence of the loop-former also entails a considerable increase in the length of the rolling line.
In the absence of corrective measures, the deformations and neck-downs are propagated along the rolled product and also affect unevenly the segment of material disposed between two successive stands.
This entails an unacceptable decrease in the quality of the rolled product which, sometimes, causes a large quantity of product to be discarded because it does not meet the dimensional tolerances required by the market.
In the state of the art, in order to solve this problem, various methods and devices have been proposed to control the inter-stand drawing which in some cases have given only partial results, not always satisfactory in terms of accurate and constant results, whereas in other cases they have entailed considerable complexities connected to the detection, command and control devices.
For example, U.S. Pat. No. 4,607,511 is known, in which the inter-stand drawing is controlled using a device to measure the diameter of a rolled bar passing through, located downstream of the rolling stands between which the control has to be carried out.
If there is a deviation in the expected nominal diameter, as detected by the diameter measurer, a control and command unit intervenes in order to modify the speed of rotation of the rolling rolls so as to modify the inter-stand drawing and to restore the correct conditions.
The presence of diameter measurers disposed upstream and downstream of the rolling blocks allows to detect deviations in size with respect to the nominal diameter as expected at exit from the rolling blocks, but it does not allow to identify the amount of such deviations, which derives from an incorrect inter-stand drawing with respect to the total of deviations detected.
A device to control the inter-stand drawing is also known and described in EP-A-0756906, in the name of the present Applicant, in which the inter-stand drawing is controlled and determined by correlating the detection data of two dimensional detectors disposed in an inter-stand segment. In particular, a first detector is disposed immediately at exit from one stand disposed upstream and a second detector is disposed immediately at entry to the stand disposed downstream.
By correlating the values obtained by the two detectors it is possible to obtain quite precise information on the regularity of the inter-stand drawing of the rolled product, since this correlation allows to identify inadmissible deformations or reductions in the section area, deriving from an irregular inter-stand drawing, not constant in the inter-stand segment. In fact, by detecting a characteristic size of the rolled product, such as for example the section or diameter, at two distinct points of the inter-stand segment, it is possible to determine an optimum inter-stand drawing condition corresponding to a rolled product with a characteristic size that corresponds to a reference size.
However, this solution has the disadvantage that it does not allow to determine possible rolling or drawing irregularities which are generated inside the stand, nor to identify the reasons that cause such irregularities, so that, if there are such anomalies, it does not allow to intervene with a retrospective command to restore the correct rolling and drawing conditions.
To overcome this disadvantage EP-A-0920926 is also known, again in the name of the present Applicant; this document has a solution similar to that in EP'906, with the difference that the data detected by the two detectors are sent to a control unit which, according to the data memorized in tables relating to the expected size of the rolled product at all points of the rolling line, determines actions on the drive members of the rolling stands to take the rolled product to an optimum drawing condition.
In both EP-A-0756906 and EP-A-0920926 , if a deviation of the inter-stand drawing from the optimum condition is detected, the action to correct the size of the rolled product can only be taken in rolling stands disposed downstream of the inter-stand segment; this entails large quantities of waste when the detectors are disposed in proximity to the terminal segment of the rolling line.
Furthermore, in order to be able to control the inter-stand drawing it is necessary to correlate the measurements made in at least two points of the inter-stand segment that have to be disposed at a determinate distance between them in order to obtain a sufficiently accurate detection of the drawing action.
One purpose of the present invention is to perfect a method to control the section size, or at least one size, of a rolled product by adjusting the inter-stand drawing, which is reliable, economical and functional, based on the adjustment of the speed of the individual rolling stands.
Another purpose of the present invention is to perfect a method that allows to reduce rolling waste and to intervene directly already in the inter-stand segment which is subjected to detection, in order to be able to carry out actions to correct possible deviations in the size of the rolled product.
Another purpose of the present invention is to achieve a control device that is extremely simple in its configuration and is economical to produce and manage.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.